Drying of some Philippine and Indonesian puddled rice soils following surface drainage: Numerical analysis using a swelling soil flow model

Heavy clay soils used for paddy rice production in SE Asia are usually pudd led to grow the rice crop. Soil conditions following puddling and drying mi ght be unsuitable for following dry season crops, such as mungbean or soybe an, which often produce poor or erratic yields. We analysed water movement in the drying process in order to better understand the drying behaviour in order that we may generalise the findings, and to examine the prospects fo r using a swelling soil flow theory in predicting that behaviour. A numeric al solution to a general, material coordinate-based, one-dimensional flow e quation for swelling soils was used to analyse experimental results from fo ur sites in Indonesia and the Philippines. The soils at the four sites were a Ustic Epiaquert, a Typic Ustropept, an Aeric Tropoquept and a Chromic Ep iaquert. In the experiments, evaporation, cracking and changes to moisture ratio (ratio of volume of moisture to volume of solid) and void ratio were monitored for several weeks following surface drainage. Hydraulic propertie s were also measured. The numerical predictions of changes to moisture ratio compared well to the measured results. Predictions of cracking were based on assumptions about the ratio of vertical to lateral shrinkage, and reasonable comparisons were obtained with the measurements. The analysis showed that the measurement o f soil evaporation by mini-lysimeters provided a good estimate of the overa ll soil evaporation at the wet end bur underestimated evaporation from dry soil. The analysis also showed that the measurernents of void ratio in the experiments were sometimes in error, because of difficulties in accurate vo lume sampling of very weak, wet soils. It is shown that in these swellings systems it was better to use material coordinates and moisture ratios rathe r than spatial coordinates and volumetric moisture contents. (C) 2000 Elsev ier Science B.V. All rights reserved.